Known power transformers, for example, those with a power rating of a few MVA and in a voltage range of, for example, 5 kV to 30 kV or 110 kV, sometimes even up to 170 kV, are also designed as dry-type transformers, wherein, in the last-mentioned voltage range, power ratings of 50 MVA and above are also entirely possible. The high-voltage-side windings can be insulated by a mixture of glass roving and epoxy resin, wherein the insulation layer formed therefrom can surround the winding.
In known design implementations, a winding can be constructed from a plurality of winding modules, e.g., from a plurality of axially adjacent hollow-cylindrical winding segments, which can be galvanically connected to one another radially on the inside and are therefore connected electrically in series. As a result, the voltage stress between radially adjacent winding layers can be reduced, and therefore so is the specified level of insulation complexity. However, this means that there is an increased differential voltage between axially adjacent winding modules at their end faces during operation of the winding, and this increased differential voltage results in increased stress on the insulation layer located therebetween. A corresponding voltage stress can also occur, however, when galvanically isolated windings are arranged axially adjacent to one another. The insulation material as such can be dimensioned readily such that it withstands this voltage stress in its interior.
In the context of the present disclosure, a hollow-cylindrical winding is understood to mean not only a winding with a circular cross section but also the term can include an approximately rectangular cross section with rounded corners. In this way, when installing the winding on a transformer core with a rectangular cross section, optimum use is made of the winding window available.
However, one disadvantage of the known implementations involves the building up of a potential difference along the outer face of the insulation layer of the winding, and in under exemplary conditions over the region of the axially adjacent winding segments. This condition can result in undesired discharges or partial breakdowns along the outer face of the common insulation layer of the winding, which discharges or breakdowns is also promoted by any contamination of the outer face.